1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member, and more particularly to an electrophotographic photosensitive member containing macromolecular binders of particular molecular weights respectively in the charge generation layer and in the charge transport layer.
2. Related Background Art
There are recently proposed electrophotographic photosensitive members of a laminate structure in which the photosensitive layer is functionally divided into a charge generation layer and a charge transport layer. These photosensitive members have been improved in the sensitivity to visible light, charge retentive force, surface strength etc. as disclosed in the U.S. Pat. Nos. 3,837,851 and 3,871,882.
Such photosensitive member of separated functions is composed at least of a charge generation layer and a charge transport layer. In such photosensitive member, the charge carriers generated by light absorption in the charge generation layer are injected into the charge transport layer and migrate to the surface thereof, thus dissipating the surface charge of the photosensitive member and creating an electrostatic contrast.
For said process of electrostatic image formation, it is required that the charge generation layer is capable of uniformly generating abundant charge carriers and efficiently injecting thus generated charge carriers into the charge transport layer, and that the charge transport layer can cause efficient migration of the charge carriers, injected from the charge generation layer. As the charge generation layer is basically a mixture of an organic pigment serving as a charge generating material and a binder while the charge transport layer is basically a mixture of an organic dye serving as a charge transporting material and a binder, the electrophotographic characteristics of the photosensitive member, such as sensitivity, potential characteristics and durability are significantly influenced by the basic structure, functional groups, molecular weight, purity etc. of the polymer compound employed as the binder.
However, though research has been made on the structure of the binder of the charge transport layer principally for improving the durability, research has scarcely been made on the binder for the charge generation layer except for the dispersibility and the adhesion property. Also there has scarcely been investigated the influence of the binder on the interaction between the charge generation layer and the charge transport layer. Consequently the conventional electrophotographic photosensitive member of separated function type have not been satisfactory in potential characteristics such as retentitive potential and photomemory performance.
In recent years, there have been increasingly proposed technologies utilizing polycarbonate resins as the binder for the charge generation layer and the charge transport layer as disclosed in the Japanese Laid-open Patent Nos. 59-227597 and 61-137157, and these technologies have resulted in a certain improvement in the retentive potential and photomemory performance of the photosensitive member.
However, if such polycarbonate resins are used as binders for both the charge generation layer and the charge transport layer, a part of the charge generation layer is dissolved into the coating solution for the charge transport layer when it is coated on the charge generation layer already coated, and a loss in sensitivity results from such dissolving of the charge generation layer into the charge transport layer. Also, in the mass production of the electrophotograhic photosensitive members, it is difficult to mass produce the photosensitive members in uniform quality, since the materials of the charge generation layer are gradually accumulated in the coating solution for the charge transport layer.
For avoiding such dissolution, there is considered a method of immersing a substrate after the coating of the charge generation layer momentarily in the coating solution for the charge transport layer and pulling up said substrate before the dissolution of the charge generation layer starts, or a spray coating method, or a coating method by pushing up a cylindrical substrate. Though these methods are suitable for production of a small quantity, they are not suitable for mass production, for example, because they require a number of equipment several times larger than that of the equipment for ordinary dip coating.